Looking back at Fig.8.2c, we see there are no negative dB
values. Could this be right? Could the spectral magnitude at all
frequencies be 1 or greater? The answer is no. To better see the true
spectrum, let's use zero padding in the time domain (§7.2.7)
to give ideal interpolation (§7.4.12) in the frequency domain:

Figure 8.4 shows the zero-padded data (top) and corresponding
interpolated spectrum on linear and dB scales (middle and bottom,
respectively). We now see that the spectrum has a regular
sidelobe structure. On the dB scale in Fig.8.4c,
negative values are now visible. In fact, it was desirable to
clip them at dB to prevent deep nulls from dominating the
display by pushing the negative vertical axis limit to dB or
more, as in Fig.8.1c (page ). This
example shows the importance of using zero padding to interpolate
spectral displays so that the untrained eye will ``fill in'' properly
between the spectral samples.